The present invention relates to a process for post-treating roughened and anodically oxidized aluminum with aqueous solutions of an alkali metal silicate. The treated aluminum is particularly useful as a support material for offset-printing plates.
Support materials for offset-printing plates are provided, on one or both sides, with a radiation-sensitive coating (reproduction coating). The coating is provided either directly by the user or by manufacturers of precoated printing plates. This coating permits the photomechanical production of a printing image of an original. Following the production of this printing form from the printing plate, the coating support has image areas which are ink-receptive during the subsequent printing steps. Also, simultaneously with image-production, a hyrophilic image-background for lithographic printing is formed in the areas which are free from an image (non-image areas).
A coating support for reproduction coatings used in the manufacture of offset printing plates must meet the following requirements:
Those portions of the photosensitive coating which have become comparatively more soluble following exposure must be capable of being easily removed from the support, by a developing operation, in order to produce the hydrophilic non-image areas without leaving a residue. PA1 The support, which has been laid bare in the non-image areas, must possess a high affinity for water, i.e., it must be strongly hydrophilic, in order to accept water, rapidly and permanently, and to repel greasy printing ink during the lithographic printing operation. PA1 The photosensitive coating must exhibit an adequate degree of adhesion prior to exposure, and those portions of the coating which print must exhibit adequate adhesion following exposure. PA1 Adding surfactants containing non-ionic and anionic moieties and, as optional ingredient, gelatin to an aqueous silicate solution used in an immersion treatment for aluminum printing-plate supports, and subsequently heating the supports, according to Japanese Published Applications No. 55,109,693) published Aug. 23, 1980) or No. 55,082,695 (published June 21, 1980); PA1 adding a combination of non-ionic and anionic surfactants to aqueous alkali metal silicate solutions used in an immersion treatment for aluminum printing plate supports, at temperatures ranging from 80.degree. to 100.degree. C., according to French Pat. No. 1,162,653; PA1 adding water-soluble organic polymers, such as, for example, polyvinyl alcohol, polyacrylic acid, polyacrylamide, polysaccharides or polystyrene sulfonic acid, to aqueous alkali metal silicate solutions used in an immersion treatment for aluminum at a temperature exceeding 40.degree. C., according to European Published Application No. 0,016,298, this treatment being especially applicable to aluminum containers; PA1 using a three-step process for producing a hydrophilic adhesive layer on aluminum printing plate supports according to German Auslegeschrift No. 1,118,009 (corresponding to U.S. Pat. No. 2,922,715), comprising the steps of (a) a chemical or mechanical roughening treatment, (b) an immersion treatment at a temperature above 85.degree. C. in an aqueous alkali metal silicate solution, and (c) a final immersion treatment at room temperature in an aqueous solution of citric or tartaric acid, in order to neutralize the alkali produced in step (b); PA1 subjecting silicate layers on aluminum printing-plate supports, which layers were produced by an immersion treatment in aqueous alkali metal silicate solutions, to a hardening after-treatment in an aqueous solution of Ca(NO.sub.3).sub.2 or, generally, in a solution of an alkaline earth metal salt, according to U.S. Pat. Nos. 2,882,153 and 2,882,154, using, as a rule, concentrations of alkaline earth metal salt above 3% by weight, the support materials being only chemically or mechanically roughened, without anodic oxidation treatment; PA1 using a process according to German Offenlegungsschrift No. 2,223,850 (corresponding to U.S. Pat. No. 3,824,159) for coating aluminum moldings, sheets, castings, or foils (for use, inter alia, as offset printing plates, but especially for use in capacitors). This process comprises an anodic oxidation in an aqueous electrolyte composed of an alkali metal silicate and an organic complex-forming compound. Such compounds include amines, amino acids, sulfonic acids, phenols, glycols and, additionally, salts of organic carboxylic acids, for example, maleic acid, fumaric acid, citric acid, or tartaric acid; or PA1 using a process for producing grain-like or textured surfaces on aluminum, according to German Auslegeschrift No. 2,651,346 (corresponding to British Pat. No. 1,523,030), which process is carried out directly on the aluminum, using an alternating current in an electrolyte which contains, in an aqueous solution, from 0.01 to 0.5 mol/l of a hydroxide or salt of an alkali metal or alkaline earth metal (e.g., a silicate and, optionally, from 0.01 to 0.5 mol/l of a compound which forms a barrier layer. The reference discloses that compounds that form barrier layers include, among others, citric acid, tartaric acid, succinic acid, lactic acid, malic acid or the salts thereof. PA1 The direct current sulfuric acid process, in which anodic oxidation is carried out in an aqueous electrolyte which conventionally contains approximately 230 g of H.sub.2 SO.sub.4 per 1 liter of solution, for 10 to 60 minutes at 10.degree. to 22.degree. C., and at a current density of 0.5 to 2.5 A/dm.sup.2. In this process, the sulfuric acid concentration in the aqueous electrolyte solution can also be reduced to 8 to 10% by weight of H.sub.2 SO.sub.4 (about 100 g of H.sub.2 SO.sub.4 per liter), or it can also be increased to 30% by weight (365 g of H.sub.2 SO.sub.4 per liter), or more. PA1 The "hard-anodizing process" is carried out using an aqueous electrolyte, containing H.sub.2 SO.sub.4 in a concentration of 166 g of H.sub.2 SO.sub.4 per liter (or about 230 g of H.sub.2 SO.sub.4 per liter), at an operating temperature of 0.degree. to 5.degree. C., and at a current density of 2 to 3 A/dm.sup.2, for 30 to 200 minutes, at a voltage which rises from approximately 25 to 30 V at the beginning of the treatment, to approximately 40 to 100 V toward the end of the treatment.
Suitable base materials for coating supports of this kind include aluminum, steel, copper, brass, or zinc foils. Plastic sheets or paper may also be used. By appropriate modifications, such as, for example, graining, matte chromium-plating, surface oxidation, and/or application of an intermediate layer, these base materials are converted into coating supports for offset-printing plates. The surface of the base material, presently most frequently aluminum, is roughened according to known methods, e.g., dry-brushing, slurry-brushing, sandblasting, or chemical and/or electrochemical treatment. In order to increase resistance to abrasion, the roughened substrate may additionally be treated in an anodizing step to produce a thin oxide layer.
In practice, the support materials, and particularly anodically oxidized aluminum-based support materials, are often subjected to a further treatment step, before applying a photosensitive coating, in order to improve the adhesion of the coating, increase the hydrophilic properties of the support material, and/or improve the developability of the photosensitive coatings. Such treatments are, for example, carried out according to the following methods:
German Pat. No. 907,147 (corresponding to U.S. Pat. No. 2,714,066), German Auslegeschrift No. 1,471,707 (corresponding to U.S. Pat. Nos. 3,181,461 and 3,280,734), and German Offenlegungsschrift No. 2,532,769 (corresponding to U.S. Pat. No. 3,902,976) describe processes for hydrophilizing support materials for printing plates made of aluminum which has optionally been anodically oxidized. In these processes, the materials are treated, with or without the application of an electrical current, with an aqueous solution of sodium silicate.
German Pat. No. 1,134,093 (corresponding to U.S. Pat. No. 3,276,868) and German Pat. No. 1,621,478 (corresponding to U.S. Pat. No. 4,153,461) describe the use of polyvinyl phosphonic acid or copolymers based on vinyl phosphonic acid, acrylic acid, and vinyl acetate to hydrophilize support materials for printing plates, comprising aluminum which has optionally been anodically oxidized.
Although these post-treatment methods often yield adequate results, they cannot meet all of the frequently very complex requirements which are demanded of a support material for printing plates, and which comprise the present standards for high-performance printing plates used in practice.
Thus, for example, upon treating the supports with alkali metal silicates which produce a good developability and good hydrophilic properties, a certain deterioration of the storability of the applied reproduction coatings must be accepted. In supports which are treated with water-soluble organic polymers, the good solubility of these polymers, particularly in the aqueous-alkaline developers which are commonly used for developing positive-working reproduction coatings, leads to a decrease in the hydrophilizing action of the post-treatment. In addition, resistance to alkali, which is particularly necessary when high-performance developers are used in the field of positive-working reproduction coatings, is not present to a sufficient degree. Depending on the chemical compositions of the reproduction coatings, tinting in the non-image areas is occasionally encountered. This tinting is probably caused by absorptive effects.
Various modifications of the silicating processes have been described previously. These modifications include, for example:
However, these known modifications of silication, anodic oxidation, or surface texturing processes using electrolytes which contain organic acids or the salts thereof, even when they are applicable to aluminum printing plate supports at all, do not produce a surface which is suitable for high-performance printing plates, i.e., technologically, the silicate layers are not improved to such an extent that they fully meet the above-indicated requirements.
German Auslegeschrift No. 2,364,177 (corresponding to U.S. Pat. No. 3,860,426) discloses a hydrophilic adhesion-promoting layer for presensitized lithographic printing plates, which is present on an anodically oxidized aluminum support and comprises a water-soluble salt of Zn, Ca, Mg, Ba, Sr, Co or Mn, in addition to a cellulose ether, for example, sodium carboxymethyl cellulose or hydroxyethyl cellulose. Such adhesion-promoting layers are intended to impart a longer useful life to the plate and to prevent "scumming" in the non-image areas during printing with a printing form produced from this plate. An appreciable increase of the resistance to alkali is, however, not obtained by means of this layer.
In German Offenlegungsschrift No. 3,219,922, a process for post-treating roughened and anodically oxidized aluminum supports for printing plates is described. In this process, an aqueous alkali metal silicate solution of the above-mentioned kind is used, additionally containing an aliphatic monobasic, dibasic or tribasic hydroxycarboxylic acid, an aliphatic dicarboxylic acid, or a water-soluble salt of these acids.